The present invention relates to an apparatus for determining the operational state of an extruder that is used for the production of a plastic profile, with the extruder comprising an extrusion cylinder with at least one endless screw, an adapter part and an extrusion die which form a flow channel for the melt and with at least one measuring device for determining the quality of the melt being provided downstream of the endless screw.
An extruder for producing plastic profiles as are required for the production of plastic window frames for example are usually designed in such a way that an adapter part is provided adjacent to the extruder cylinder in which the endless screws are disposed, which adapter part forms the transition to the extrusion die. The cross-sectional progress of the respective profile is already substantially shaped in the extrusion die in order to be finally determined in the subsequent dry and/or wet calibration. In order to increase the productivity of the extrusion process, processing is carried out with ever increasing extrusion speeds. At the same time, demands placed on the quality are also increasing. A high quality of the profiles can be achieved at higher extrusion speeds only with an extremely careful calibration of the extrusion tools with simultaneous adherence to a precisely defined operational state in the extruder.
In order to detect the operational state of the extruder it is known to detect the pressure and the temperature of the melt in the region of the adapter part. For this purpose a pressure sensor and a temperature sensor are disposed in the zone of the flow channel of the melt for example. This allows detecting the operational state of the extruder. In the production of a new extrusion die, the calibration is performed by the manufacturer of the die. It is proceeded in such a way that the die is provided upstream with an extruder which is the same or similar to the extruder with which the die is operated which is the same or similar to the extruder with which the die is operated after delivery by the customer. The relevant aspect for the calibration is also the use of the same basic material as in later production.
It has been noticed that at high extrusion speeds or during the extrusion of profiles with low wall thicknesses and generally under high requirements placed on the quality of the profile, such as in the case of narrow tolerances, only unsatisfactory results are achieved even in the case of optimal calibration of the die at the manufacturer after the delivery of the die to the customer. The reason is obviously that even extruders of the same design show slight differences which are caused by production tolerances, wear and tear of the like. The inventors of the present invention have recognized that such differences can be present even in cases where the measurements in the adapter part show identical values for temperature and pressure. In the inventors' opinion this is linked to the fact that the rheological properties of the plastic melt cannot be sufficiently described with the parameters pressure and temperature. One consequence of this fact is that after the assembly of the extrusion die at the customer's location, it is necessary to perform work-intensive calibration drives during which it is attempted to eliminate determined deficiencies in the profile by changing the extrusion conditions, such as the screw speed, heating output and the like. Since it is very difficult to find out which deviation is responsible for the occurrence of a certain deficiency, such as deteriorated surface quality in a specific partial zone of the profile, such work is usually labor-intensive and cumbersome.
It is further known to flange so-called rheometer nozzles on an extruder, which nozzles are arranged as slotted nozzles. The material data thus obtained have proven to offer insufficiently meaningful information, especially in connection with the material of PVC, for the actual extrusion process, which means that a fine adjustment on the basis of the data thus gained is not possible.
From DE 36 42 757 A, a measuring apparatus is known in which rheological properties of an extrusion material are performed by the measurement of the pressure drop in the main flow. Such a method is possible when lines of individual spinnerets start out from an extruder. Such measuring methods cannot be used in a satisfactory way in extrusion apparatuses in which an extrusion die is provided directly next to an extruder.
EP 0 899 556 A relates to a measuring apparatus in which measurements are performed on the basis of a material flow which is gained directly from the extruder. It has been noticed, however, that measuring results are obtained which are not very meaningful.